Optical waveguides capable of transmitting power with only one direction of polarization are desirable for use with integrated optical devices. However, it is well known that geometric or dielectric imperfections in conventional graded-index fibers depolarize light after only a few centimeters of propagation. While some slight improvment in the polarization performance of these fibers is achieved by distorting the fiber core symmetry as a means of decoupling the differently polarized waves, and analysis, based on an article by E. A. J. Marcatili entitled "Dielectric Rectangular Waveguide and Directional Coupler For Integrated Optics" published in the September 1969 issue of the Bell System Technical Journal, pp. 2071-2102, shows that simply changing the core geometry does not appreciably alter the difference in the propagation constants of the two orthogonally-polarazied fundamental modes.
An alternative approach to this problem is disclosed in U.S. Pat. No. 3,659,916 which discloses a fundamental mode strip waveguide in which a lossy material is placed along one surface of the guiding strip to suppress one of the two orthogonally polarized modes. Alternatively, a higher refractive index material can be used instead of a lossy material as a means of destroying the waveguide's ability to guide one of the modes. While these techniques serve to suppress one of the two modes by absorption or radiation, they do not preclude coupling between the modes. As a result, there is a constant draining of power from the preferred polarization to the undesired polarization, and consequent loss. Thus, single polarization waveguides of the type described tend to be unduly lossy.
In a copending application by Pleibel and Stolen, Ser. No. 869,365, filed concurrently with this application on Jan. 13, 1978, an improved single polarization fiber is disclosed comprising an outer layer having a circular outer surface and an elliptical inner surface; an inner elliptical core region; and an intermediate cladding layer that conforms to the core and substrate surface configurations. While it was initially believed that the improved polarization-preserving characteristics were due to the geometrically asymmetric configuation, it has been shown that stress-induced birefringence is, in fact, the dominant factor in the improved polarization performance of this particular fiber structure. While stress-induced birefringence, of a magnitude on the order of 10.sup.-7, has been measured in conventionally drawn optical fibers (see "Birefringence in Dielectric Optical Waveguides," by F. P. Kapron et al., published in the IEEE Journal of Quantum Electronics, Vol. QE-8, No. 2, February 1972, pp. 222-225), the size of the effect is insufficient to reduce polarization coupling to a practically useful level. Furthermore, stress-induced birefringence was not recognized as a possible means for reducing such coupling.